The present invention relates to an improved electrochemical method for forming an inorganic covering layer, having specified optical, chemical and mechanical characteristics, on a surface of a copper material; the invention further relates to a copper article, made of a copper or copper-based alloy material at least in a surface thereof, comprising an inorganic covering layer having specified optical, chemical and mechanical characteristics.
It is known that copper or copper-based alloy materials require, in many different applications, to be covered by a layer having specific characteristics. Different applications usually require different features of the covering layer: for example, good electrical insulating properties are essential in wire protection, optical properties (reflectance, gloss, etc.) are vital in solar energy collectors, a particular aesthetic appearance and a particular colour are important in roofing and building construction in general; in any case, the layer should be as adherent as possible to the copper material (in order to avoid a possible detachment during the production process or in use) and have good mechanical characteristics.
A variety of methods has heretofore been proposed for the formation of different kinds of coating layers on copper material surfaces. In particular, chemical conversion or anodizing process are known. In chemical conversion, the copper material to be treated is dipped at a high temperature in an electrolytic bath comprising an alkali salt at a high concentration and an oxidizing agent, in order to form a copper(II) oxide (cupric oxide, CuO) layer: this method requires not only a long process time but also a rather high cost for the reagents and its productivity is therefore poor. In traditional anodizing (i.e. anodic oxidation), a layer composed of copper(II) oxide CuO is formed on a copper material at a high current density in a alkaline solution of a high concentration: since CuO thus formed is instantaneously redissolved even by a slight variation in process conditions (alkali concentration, current density), the process control is extremely difficult.
Improved anodizing processes are disclosed in U.S. Pat. Nos. 5,078,844 and 5,401,382: a tough electrically insulating layer is formed on a copper surface by anodizing at low current density in an acidic electrolytic bath of a hexacyanoiron complex; eventually, such an anodic oxidation process may be preceded by a further anodizing in an alkaline electrolyte bath of a caustic alkali to form a thin film layer of cupric oxide CuO. These methods provide efficient production of insulating composite layers made of copper(I) oxide and copper hexacyanoferrate(I) or (II), but they require relatively long process time and cost and, furthermore, the layers are not suitable for many applications, in particular where specified colour, aesthetic appearance and optical features are required. It is also known from EP-A-751233 a browning method for forming a dark brown covering layer on a copper strip or sheet to be used, for example, in roof covering: the copper material is heat-treated at high temperature (250÷750xc2x0 C.) in a mixed gas atmosphere containing O2 to form a cuprous oxide (Cu2O) layer conditions or, alternatively, treated with an aqueous alkaline solution to form a cupric oxide (CuO) layer: in both cases the final covering layer consists of a first layer made of Cu2O adhering to the base metal and a second layer made of CuO over the first layer. This method requires a high contact time in order to form the composite covering layer and it also has relatively high production costs, basically due to a reduced chemical stability of some oxidants and to higher costs of waste treatment, which are essential in order to avoid water and air pollution.
From GB 1052729 a method is known of anodic oxidation of the surface of a copper sheet in an alkaline aqueous solution, e.g. a solution of NaOH or KOH, under certain process conditions but, always at current densities below 0.5 A, thus resulting only in a predominantly golden-yellow to golden red coating Cu2O.
This patent do not provide a method of anodic oxidation to form a continuous homogeneous layer formed of copper (I) oxide (cuprous oxide, Cu2O) to obtain different colours by varying specific parameters.
It is an object of the present invention to provide an improved electrochemical method for forming a covering layer on a copper surface which allows the aforementioned disadvantages connected with the known processes to be eliminated. In particular, it is object of the present invention to provide a fast, simple and inexpensive method which is really effective in providing a copper surface with a coating layer having specified colour, aesthetic appearance and optical properties, improved adherence to the copper substrate and excellent mechanical characteristics by varying the current densities.
According to the present invention, there is provided an electrochemical method for producing an inorganic covering layer on a surface of a copper or copper-based alloy element, characterized in that it comprises an anodic oxidation of said element, in which said surface is placed in an alkaline electrolytic bath of at least an alkali in aqueous solution under controlled process conditions of temperature and current density and for a period of time suitable to form a continuous substantially homogeneous layer on said surface, said layer being basically formed of copper(I) oxide (cuprous oxide, Cu2O) having a crystalline structure and a consequent colour determined by said controlled process conditions. In particular, the anodic oxidation is performed in a bath containing only a single alkali salt in aqueous solution, preferably at a concentration of about 1,25÷11,25 mole/l of hydroxyl ion (for example, sodium hydroxide NaOH at a concentration of about 50÷450 g/l or potassium hydroxide KOH at a concentration of about 70÷630 g/l).
Preferably, the anodic oxidation is performed in a bath containing at least 20% wt. sodium hydroxide in aqueous solution, at a temperature of at least 60xc2x0 C., at a current density of 0,5÷20 A/dm2 with a cell potential (anode/cathode potential difference) of about 0,2÷3 V and for a period of time of about 5÷120 seconds. More preferably, the anodic oxidation is performed in a bath of 30% wt. NaOH in aqueous solution, at a temperature of about 82xc2x0÷92xc2x0 C. and for a period of time of about 10÷30 seconds, suitable for obtaining a layer having a thickness of about 0,100÷0,400 xcexcm. The final colour of the layer is determined by varying the current density in said alkaline electrolytic bath in the range 0,3÷20 A/dm2: in particular, the anodic oxidation is performed at a current density of about 0,5÷1 A/dm2 to form a layer having a brown colour, at a current density of about 2,5÷3 A/dm2 to form a layer having a dark brown colour, at a current density of about 10÷15 A/dm2 to form a layer having a deep black colour and a velvety surface effect.
The method according to the invention may further comprise, before the anodic oxidation process, a pre-treatment step of the copper surface (for example a chemical or thermal pre-oxidation, a skin passing process, a benzotriazole inhibition, etc.), as well as, after the anodic oxidation, a final surface treatment (for example a further skin-passing step).
According to an embodiment of the invention, the anodic oxidation starts as soon as the copper surface is placed in contact with the electrolytic bath; as an alternative, the anodic oxidation starts a period of time of about 3÷180 seconds after the copper surface is placed in contact with the electrolytic bath.
In case the copper or copper-base alloy element is a plate-like element, both surfaces of said element may be simultaneously subjected to the anodic oxidation according to the invention. Clearly, the method of the invention is absolutely not limited to a plate-like article, but it can be applied to articles having substantially any shape and geometry.
The present invention further relates to a copper article, made of a copper or copper-based alloy material at least in a surface thereof, characterized in that it comprises a continuous substantially homogeneous covering layer (preferably having a thickness of about 0,100÷0,400 xcexcm) on said surface, said layer film being basically formed of copper(I) oxide (cuprous oxide, Cu2O) having a defined crystalline structure for giving to said surface specific optical properties.
The basis of the present invention is therefore to be seen in conducting, in a traditional electrolytic cell, an anodic oxidation of a copper surface under controlled condition, so as to form basically cuprous oxide Cu2O and avoid the formation of cupric oxide CuO: the process of the invention can be therefore considered an improved electrochemical browning process which overcomes all the drawbacks of the above-mentioned known processes: in fact, only an alkaline solution (for example an aqueous sodium hydroxide solution, extremely cheap) and electrical current (with a relatively low consumption) are used in the method according to the invention: the desired layer is produced by electrochemistry avoiding the use of chemical oxidants and/or other expensive reagents: just a very short contact time is required (significantly shorter than in known processes), so that the whole process is faster, simpler and less expensive than known processes. The method can be defined a xe2x80x9czero-wastexe2x80x9d process, since the consumption of chemicals involved in the process is extremely low and may be further limited by using, for example, vacuum or inverse osmosis concentrators. Also environmental impact is reduced when compared with known processes.
Any known electrochemical apparatus may implement the method according to the invention: as previously stated, the electrolytic bath contains only an alkali, preferably sodium hydroxide (very inexpensive), and demineralized water: no additives or other chemicals are required, as in other known browning processes.
The coloured final layers are basically made of pure crystalline copper(I) oxide Cu2O containing traces of copper(II) oxide CuO, as revealed by chronopotentiometric analysis (a branch of coulometry). Cu2O layers offers, when compared to CuO layer, many advantages: they are more adherent to the copper substrate and show a better mechanical behaviour, allowing any detaching problem to be avoid, both during mechanical treatment and in use; they show a better leaching resistance to acid rains and to atmospheric conditions in general; they have superior optical properties and a more pleasant aesthetic appearance, being able to assume a brown colour with desired intensity and shade; they have good insulating properties, realizing an effective barrier to metallic diffusion, and are scarcely influenced by heat and nuclear radiation. The method according to the invention allows different colours to be obtained, simply controlling the anodizing process condition (mainly current density) and consequently the crystalline form of the oxide layer: even a velvety deep black layer may be obtained, showing improved optical properties when compared, for example, to black chromium (which is traditionally used, as well known, to cover copper sheets by electroplating for solar energy applications).
The method according to the invention is particularly useful to produce very uniform coloured thin layers on copper continuous strips and single sheets (on both sides thereof), but it may also be used on copper articles having any shape: the method may be easily implemented either in a continuous or in a batch industrial process. The method of the invention may be applied to any kind of copper base material: for example, pure copper, copper-base alloys (brasses and bronzes), either pre-treated (e.g. thermally or chemically pre-oxidized, skin-passed, benzotriazole inhibited) or not: no pre-cleaning treatments, as degreasing and pickling, are necessary. Copper surfaces realized according to the present invention can be used in a wide range of industrial applications, thanks to the above described favourable features: in particular (but not only), the method of the invention allows an easy, fast and inexpensive production of rolled strips or sheets for roofing (standard or energetic roofing), ornamental articles to be used in building construction, pre-oxidized copper foils for electronics or oxidized copper surfaces of inner layers in electronic circuits (blackening), heat collectors for solar energy applications (instead of traditional high expensive black chromium electroplated copper surfaces); the method can also be used for covering cylindrical geometry surfaces, such as wire, bars, tubes and mineral insulated electrical cables, with copper sheathing, as well as for in electrical insulation of copper wires and superconductor cables, in which case the anodizing process may be prolonged for a time suitable to produce CuO instead of Cu2O.
As previously stated, the anodizing process according to the invention may be preceded by a pre-treatment, for example a pre-oxidation: it has been noted that copper substrate surface quality influences the oxide layers formation, in term of chemical composition and crystalline form as well as in term of thickness.
Also pre-conditioning time (i.e. the time between the moment in which the copper article enters the alkaline bath and the moment in which it arrives in the applied electrical field) may be used for influence layer quality: when the pre-conditioning time is increased, final aspect and colour of the layers are improved.